Structure with switchable magnetic properties

ABSTRACT

A structure ( 40 ) with switchable magnetic properties comprises: an array of capacitive elements ( 44 ) in which each capacitive element ( 44 ) includes a low resistance conducting path and is such that a magnetic component (H) of electromagnetic radiation ( 12 ) lying within a predetermined frequency band induces an electrical current (j) to flow around said path and through said associated element ( 44 ). The size of the elements ( 44 ) and their spacing (α) apart are selected such as to provide a predetermined permeability (μ) in response to said received electromagnetic radiation ( 12 ). Each capacitive element ( 44 ) comprises a plurality of stacked planar sections ( 42 ) each or which comprises at least two concentric spiral conducting members or tracks ( 46, 48 ) which are electrically insulated from each other. A switchable permittivity material, such as Barium Strontium Titanate (BST) is provided between the tracks. The magnetic properties of the structure are switched by applying a dc electrical potential between the conducting tracks.

This invention relates to a structure with switchable magneticproperties.

In certain applications it is advantageous if the magnetic permeabilityof a material can be tailored for that application at least within aspecified frequency range and more especially if its magneticpermeability could be switched between selected values. In ourco-pending UK Patent Application No. 2346485 (International PatentApplication No. WO 00/41270) and in a publication entitled Magnetismfrom Conductors and Enhanced Non-Linear Phenomena, IEEE Transaction onMicrowave Theory and Techniques, 1999, 47, 2075-2084, J B Pendry, A JHolden, D J Robbins and W J Stewart, a structured material is disclosedwhich exhibits a magnetic permeability at a selected frequency,typically a microwave frequency (GHz). The content of these documents ishereby incorporated by way of reference thereto.

The structured material described in these documents comprises an arrayof capacitive elements which include a low resistance electricallyconducting path and in which the elements are arranged such that amagnetic component of electromagnetic radiation within a selectedfrequency band induces an electrical current to flow around the path andthrough the associated element. The size of the elements and theirspacing are selected such as to provide a selected magnetic permeabilityin response to the electromagnetic radiation. Such a structure allows amaterial to be fabricated which is designed to have a selected fixedmagnetic permeability for a selected frequency of electromagneticradiation.

As shown in FIGS. 1(a) and (b) one such structured material 2 comprisesan array of capacitive elements 4 each of which consists of twoconcentric metallic electrically conducting cylindrical tubes: an outercylindrical tube 6 and an inner cylindrical tube 8. Both tubes 6, 8 havea longitudinal (i.e running in an axial direction) gap 10 and the twogaps 10 are offset from each other by 180°. The elements 4 are arrangedin a regular square array and are positioned on centres at a distance aapart. The outer tube 6 has a radius r and the inner 8 and outer 6cylindrical tubes are separated by a distance d. The gap 10 prevents theflow of dc electrical current around either of the cylinders 6, 8.However the self capacitance between the two cylindrical tubes 6, 8allows an ac current, j, to flow when the material is subjected toelectromagnetic radiation 12 having a magnetic field component H whichis parallel to the axis of the tubes 6, 8. It is shown that such astructure has an effective magnetic permeability μ_(eff) (ω) which isgiven by: $\begin{matrix}{{\mu_{eff}(\omega)} = {1 - \left\lbrack \frac{\frac{\pi \quad r^{2}}{a^{2}}}{1 + \frac{2\sigma \quad i}{\omega \quad {r\mu}_{0}} - \frac{3{dc}_{0}^{2}}{\pi^{2}\omega^{2}r^{3}}} \right\rbrack}} & {{Eq}.\quad 1}\end{matrix}$

in which ω is the angular frequency, σ the resistivity of thecylindrical tubes, i the {square root over (−1)} and c₀ the velocity oflight. From Eq. 1 it can be seen that by appropriate selection of thesize r and spacing a of the cylindrical tubes a structure having aselected magnetic permeability at a given frequency ω can be obtained.

For ease of fabrication it proposed in UK Patent Application No. 2346485(International Patent Application No. WO 00/41270) to construct eachcapacitive element 4 in the form of a stack of concentric split rings26, 28 as shown in FIGS. 2(a) and 2(b). A stack of such rings is shownto be equivalent to the concentric cylindrical tubes described above andhas a magnetic permeability given by: $\begin{matrix}{{\mu_{eff}(\omega)} = {1 - \left\lbrack \frac{\frac{\pi \quad r_{1}^{2}}{a^{2}}}{1 + {\frac{2{l\sigma}_{1}}{\omega \quad r_{1}\mu_{0}}i} - \frac{3{lc}_{0}^{2}}{{\pi\omega}^{2}r_{1}^{3}{\ln\left\lbrack \quad \frac{2c_{1}}{d_{1}} \right\rbrack}}} \right\rbrack}} & {{Eq}.\quad 2}\end{matrix}$

where r₁ is the inside radius of the inner ring 28, a the latticespacing of the rings, l the separation between the rings in a givencolumn in an axial direction, d₁ the separation between the rings in aradial direction, c₁ the width of each ring in a radial direction and σ₁the resistance per unit length of each ring.

A further microstructured material described in United Kingdom PatentApplication No. 2346485 (International Patent Application No. WO00/41270) is constructed using a stack of conducting elements whichcomprise a single spiral shaped conductor 34 as illustrated in FIGS.3(a) and 3(b).

It is also suggested that in United Kingdom Patent Application No.2346485 (International Patent Application No. WO 00/41270) that themagnetic permeability of the structured material could he made to beswitchable by incorporating an non-linear dielectric medium, such asBarium Strontium Titanate (BST) or other ferroelectric material, intothe structure. The magnetic permeability of the structure is switched bychanging the permittivity of the ferroelectric material by applying anelectric field across the ferroelectric material. It is suggested thatthe ferroelectric material could be incorporated between the cylindricaltubes of each capacitive element (FIG. 1(b)) or between each of theconcentric rings in a radial direction (FIG. 2(a)). The inclusionhowever of a ferroelectric material, such as BST, decreases the resonantfrequency of the structure by a factor of more than 30 times. Toincrease the resonant frequency to a selected value to obtain thedesired magnetic permeability at a given frequency requires the selfcapacitance of each capacitive element to be reduced by the same factor.When it is intended that the structured magnetic material is to operateat microwave frequency, that is in the GHz region, this would require astructure composed of capacitive elements which were impractical tofabricate. To overcome this problem it is proposed in United KingdomPatent Application No. 2346485 (International Patent Application No. WO00/41270) that the structure comprises an array of single, rather thanconcentric, cylindrical tubes each of which has two gaps running in anaxial direction. A ferroelectric is provided in the gaps and themagnetic permeability switched by changing the permeability of theferroelectric material using an electrical static switchable electricfield. Although such a structured material is capable of operation atmicrowave frequencies it is impractical to fabricate capacitive elementssufficiently small for operation at radio frequencies in the MHz region.Furthermore even for microwave operation the construction of such astructured material is difficult and expensive.

The present invention has arisen in an endeavour to provide a structuredmaterial having a magnetic permeability which can be switched betweenselected values at a selected wavelength of operation, which can bereadily fabricated and which is suitable for operation at radiofrequencies (MHz).

According to the present invention there is provided a structure withswitchable magnetic properties comprising an array of capacitiveelements in which each capacitive element includes a low resistanceconducting path and is such that a magnetic component of electromagneticradiation lying within a predetermined frequency band induces anelectrical current to flow around said path and through said associatedelement and wherein the size of the elements and their spacing apart areselected such as to provide a predetermined permeability in response tosaid received electromagnetic radiation, characterised in that eachcapacitive element comprises a plurality of stacked planar sections eachof which comprises at least two concentric spiral conducting memberswhich are electrically insulated from each other and which have aswitchable permittivity material therebetween.

The magnetic permeability of the structure can be readily switched to aselected value by applying a static electric field across the switchablepermittivity material. This is conveniently achieved by applying a dcvoltage between the conducting spiral members of each capacitiveelement. In the context of this patent application the term spiral is tobe construed broadly and is not restricted to a plane curve which istraced about a fixed point from which it continuously recedes. The termincludes any unclosed loop of more than one turn which recedes away froma centre point. As such the term encompasses spirals which are square,rectangular, triangular, hexagonal or have other geometric forms.

Preferably the spirals are substantially circular in form. Alternativelythey are square or rectangular in form.

Advantageously the switchable permittivity material comprises aferroelectric material, preferably Barium Strontium Titanate.Alternatively it can comprise a liquid crystal.

Preferably the capacitive elements are arranged on a square array.Advantageously alternate spiral conducting members in a given row unwindin an opposite sense. With such an arrangement the structureadvantageously further comprises electrically conducting connectingtracks connecting respective spiral members in a given column.

Preferably the structure is configured for operation at radiofrequencies (MHz).

The structures of the invention are non-magnetic in a steady magneticfield.

A structure with switchable magnetic properties in accordance with theinvention will now be described by way of example only with reference tothe accompanying drawings in which:

FIG. 1(a) a schematic representation of a known structured materialhaving magnetic properties;

FIG. 1(b) an enlarged view of one of the capacitive elements of FIG.1(a);

FIG. 2(a) a schematic representation of a further known structuredmaterial having magnetic properties;

FIG. 2(b) an enlarged plan view of one of the capacitive elements ofFIG. 2(a) and a stack of such elements;

FIG. 3(a) a schematic representation of yet a further known structuredmaterial;

FIG. 3(b) an enlarged plan view of one of the capacitive elements ofFIG. 3(a) and a stack of such elements;

FIG. 4 a schematic representation, in exploded view, of a structure withswitchable magnetic properties in accordance with the invention;

FIG. 5 a plan view of one of the capacitive elements of the structure ofFIG. 4;

FIG. 6 a plan view of a single layer of the structure of FIG. 4;

FIG. 7 a plot of the real and imaginary parts of the magneticpermeability as a function of frequency for the structure of FIG. 4 inan “unswitched” and “switched” state; and

FIG. 8 a further form of capacitive element for use within a structurewith switchable magnetic properties in accordance with the invention.

Referring to FIG. 4 there is shown a structure, or structured material40, having a switchable magnetic permeability. The structure 40comprises a stack of electrically insulating sheets 42 each of which hasan array of electrically conducting capacitive elements 44 defined onits upper surface. For clarity the structure 40 in FIG. 4 is shown inexploded view with the sheets 42 separated. In practice however thesheets 42 are stacked on top of each other with the capacitive elementsof one sheet 42 overlaying the corresponding elements 44 of adjacentsheets. The capactive elements are separated by a distance l from theircorresponding neighbours on the adjacent sheet. The sheets 42 comprise aglass fibre printed circuit board or other insulating material such as apolyamide thin film and the electrically conducting capacitive elements44 are defined in the form of copper tracks using photolithography orother suitable technique.

Referring to FIG. 5 there is shown, in plan view, a single capacitiveelement 44. Each capacitive element 44 comprises two concentricelectrically conducting spiral tracks 46, 48 of N turns; five turns inthe case of the element illustrated. The two spiral tracks 46, 48 areelectrically isolated from each other and each have an inner r_(in) andan outer r_(out) radius. Each spiral track 46, 48 is of width c and thetracks separated by a distance d in a radial direction. The resistanceper unit length of the tracks is ρ. The gap running between the tracks46, 48 is filled with a dielectric paint that is based on BariumStrontium Titanate (BST) ceramic powder. For ease of fabrication the BSTpaint is applied over the whole surface of each sheet 42. This material,which is ferroelectric, has a permittivity which is large and non-linearand can be switched by the application of a static electric field. Anelectric field can be applied across the BST by applying a dc electricvoltage across the tracks 46, 48 using electrically conducting electrodetracks 50, 52. For the sake of clarity the electrode tracks 50, 52 arenot shown in FIG. 4.

Referring to FIG. 6 a single sheet 42 is shown in plan view illustratingthe layout of the capacitive elements 44 and the arrangement of theelectrode tracks 50, 52. The capacitive elements 44 are arranged on asquare array of lattice dimension α. As illustrated in FIG. 6 the spiralelements 46, 48 within each row are alternately spiraled in an oppositesense. In contrast capacitive elements within each column have the samesense. This arrangement means that the outer most conducting track 46,48 of adjacent capacitive elements are the same and can therefore beconnected to the same electrode track. Whilst it is preferred for easeof connection to arrange the elements in this way it is not essential tothe functioning of the structure and in alternative embodiments thespiral elements can have the same sense. A particular advantage of thisarrangement is that it minimises the length of the electrode tracks 50,52 required to interconnect each of the spiral tracks 46, 48 within agiven sheet 42. This has the benefit of reducing the interaction of thestructure 40 with the electric field component E when the structure issubjected to electro-magnetic radiation 12.

By assuming the width c of each spiral track is very much smaller thanthe radius of the spiral it can be shown that the magnetic permeabilityof the structure described is approximately: $\begin{matrix}{{\mu_{eff}(\omega)} \approx {1 - \frac{\pi \quad {r_{out}^{2}/a^{2}}}{1 + {\frac{\pi \quad {{{pl}\left( {N - 2} \right)}\left\lbrack {r_{in} + {\frac{1}{2}{N\left( {c + d} \right)}}} \right\rbrack}}{{\omega\pi}\quad r_{in}^{2}{\mu_{0}\left( {N - 1} \right)}^{2}}i} - \left\lbrack \frac{1}{\pi \quad r_{in}^{2}{\mu_{0}\left( {N - 1} \right)}^{2}\omega^{2}8{ɛɛ}_{0}\frac{\left( {c + d} \right)}{l}\frac{\ln \left( {2{c/d}} \right)}{\ln \left( {r_{out}/r_{in}} \right)}} \right\rbrack}}} & {{Eq}.\quad 3}\end{matrix}$

in which ε is the permittivity of the dielectric material between thespiral tracks 46, 48, ε₀ and μ₀ the permittivity and permeability offree space respectively and i={square root over (−1)}. It can be seenfrom Eq. 3 that the magnetic permeability is dependent on thepermittivity of the material between conducting spiral tracks 46, 48.Therefore the magnetic permeability of the structure can be switched toa selected value by appropriate switching of the permittivity. Asdescribed above, this is achieved by applying a potential difference −V,+V between the electrode tracks 50, 52 of each layer 42 of thestructure.

When the structured material 40 is subjected to electromagneticradiation 12 whose magnetic field H is perpendicular to the plane of thesheets 42, that is it is parallel with the axis of the capacitiveelements 44 (as shown in FIG. 4), this induces an alternating electricalcurrent in each of the conducting spiral tracks 46, 48. Since theelectrically conducting spiral tracks 46, 48 of each element 44 areinsulated from each other this prevents dc current flow. However thereis considerable self capacitance between the tracks 46, 48, especiallywhen each element 44 has a number of turns and this allows an acelectrical current flow between the inner and outer ends of the spiraltracks 46, 48. These induced electrical currents generate largeinhomogeneous electric fields within the structure 40 which gives riseto the structure's magnetic properties. It will be appreciated that themagnetic properties of the structured material arise from the selfcapacitance of the element's 44 interacting with a magnetic component ofa radiation rather than from any magnetism of its constituentcomponents. If a dc (static) voltage (−V, +V) is applied to theelectrode tracks 50, 52 this will apply an electric field across theferroelectric material thereby changing its permittivity which in turnwill change the magnetic permeability of the structured material.

An example of the performance of a structure made in accordance with theinvention is shown in FIG. 7 for a structure in which r_(in)=5 mm,r_(out)=12.1 mm, c=0.5 mm, d=0.1 mm, N=5,l=0.5 mm and α=30 mm. Thespiral tracks 46, 48 are made of copper with a resistance of 100 Ωm⁻¹.The gap between turns of the spiral tracks 46, 48 is filled with BSTwhose permittivity in an “unswitched state”, that is with no electricfield applied, is equal to 200 and in a “switched state”, that is withan electric field of 1 kVm⁻¹ applied, is 100. For the structuredmaterial 40 described such a field intensity corresponds to theapplication of 100V between the electrode tracks 50 and 52. FIG. 7 is aplot of magnetic permeability versus angular frequency in which thesolid line represents the real part of the magnetic permeability for thestructure in an “unswitched state”, that is with no voltage appliedbetween the electrodes 50 and 52, the dotted line represents theimaginary part of the magnetic permeability in an “unswitched state”,the dashed/dotted line represent the real part in a “switched” state andthe dashed line the imaginary part in the “switched state”. As will beapparent from these plots the structure of the present inventionexhibits a magnetic permeability having a resonance at radio frequencies(MHz) which can have negative values, large positive values and othervalues in between which can be selected by applying an appropriate dcpotential to the electrode tracks.

The structure of the present invention will find many applications whereit is desired to have a structure with switchable magnetic properties ata selected wavelength especially where there is a steady state magneticfield or a field gradient that should not be perturbed by the presenceof the material. One example is in the field of magnetic resonanceimaging (MRI). A structured material in accordance with the invention isparticularly suited for use in MRI machines operating at 21.3 MHz. Atthis frequency of operation a structured material can be fabricatedwhich, in the unswitched state has a negative permeability and henceacts as a screen for the radio frequency (rf) field used in suchmachines but does not affect a steady state magnetic field. In theunswitched state the material acts as a screen by reflecting rfradiation from its outer layer and additionally any radiation thatpenetrates the outer layers is rapidly attenuated. When the material isswitched, by the application of a static electric field, the materialhas small positive magnetic permeability (i.e. μ<2). For intermediateconditions the permeability may be positive or negative, large or smallgiving rise to either guiding or screening properties depending on thevoltage applied to the electrode tracks. It will be appreciatedtherefore that a structure whose permeability can be changed in realtime allows an MRI machine to be reconfigured as desired. For example asdescribed in our co-pending United Kingdom Patent Application No.0005354.6 it is proposed to use an array of sensing coils for receivingmagnetic resonance signals from a desired region of a patient. Screensmade of a structured material in accordance with the invention havingswitchable magnetic properties are provided between the coils. Byappropriate switching of the magnetic permeability of the screens theeffective region viewed by each coil can be varied.

It will be appreciated that the present invention is not restricted tothe specific embodiment described and that variations can be made thatare within the scope of the invention. Whilst the capacitive elementsare preferably in the form of two concentric circular spiral tracksother forms of capacitive elements could be used such as for example, adouble spiral which is square in form as illustrated in FIG. 8.Furthermore each spiral track or member could be any form of an unclosedloop of more than one turn such as for example triangular or othergeometric form. A particular advantage of a spiral shaped conductingtrack of a number of turns is that it is intrinsically small and has alarge self capacitance for a given size of capacitive element. Thissmall size of element enables a structured material to be fabricatedwhich is capable of operation at radio frequencies.

Furthermore in alternative embodiments it is envisaged to incorporateadditional conducting tracks into the spiral element. It will beappreciated that other ferroelectric materials could be incorporatedinto the structure such as for example a liquid crystal which could beprovided over the whole surface of each sheet.

Whilst arranging the capacitive elements in the form of a square arrayis convenient for interconnecting the respective tracks of thecapacitive element within a given sheet, the capacitive elements canalternatively be arranged in different arrays.

What is claimed is:
 1. A structure with switchable magnetic propertiescomprising: an array of capacitive elements, the array exhibiting apredetermined magnetic permeability in response to incidentelectromagnetic radiation lying within a predetermined frequency band,in which each capacitive element includes a low resistance conductingpath and is such that a magnetic component of the electromagneticradiation lying within the predetermined frequency band induces anelectrical current to flow around said path and through said associatedelement, wherein the spacing of the elements is less than the wavelengthof the radiation within the predetermined frequency band, wherein thesize of the elements and their spacing apart are selected such as toprovide the predetermined permeability in response to receivedelectromagnetic radiation, and wherein each capacitive element comprisesat least two concentric spiral conducting members which are electricallyinsulated from each other and which have a switchable permittivitymaterial therebetween.
 2. A structure as claimed in claim 1, in whichthe switchable permittivity material comprises a ferroelectric material.3. A structure as claimed in claim 1, in which the switchablepermittivity material comprises Barium Strontium Titanate.
 4. Astructure as claimed in claim 1, in which the switchable permittivitymaterial comprises a liquid crystal.
 5. A structure as claimed in claim1, in which the capacitive elements are arranged in a planar array.
 6. Astructure as claimed in claim 5, in which alternate spiral conductingmembers in a given row unwind in an opposite sense.
 7. A structure asclaimed in claim 5, including a plurality of planar arrays arranged in astack.
 8. A structure as claimed in claim 7, and further comprisingelectrically conducting connecting tracks connecting respective spiralmembers in a column of spiral conducting members.
 9. A structure asclaimed in claim 1, in which the spacing of the elements is less thanone half of the wavelength of the radiation within the predeterminedfrequency band.
 10. A structure as claimed in claim 1, in which thespacing of the elements is less than one fifth of the wavelength of theradiation within the predetermined frequency band.
 11. A structure asclaimed in claim 1, in which the spacing of the elements is less thanone tenth of the wavelength of the radiation within the predeterminedfrequency band.
 12. A structure as claimed in claim 1, in which thestructure exhibits a negative magnetic permeability over at least a partof the predetermined frequency band.
 13. A structure as claimed in claim1, in which the predetermined frequency band is within the bandextending 3 MHz to 300 MHz.
 14. A structure as claimed in claim 13, inwhich the predetermined frequency band is within the band extending from3 MHz to 30 MHz.